The present invention relates in general to providing automatic gain control for an RF amplifier in a radio receiver, and more specifically to providing automatic gain control for an RF amplifier without desensitizing the receiver to the desired radio signal and while minimizing effects of intermodulation distortion.
Automatic gain control is a well-known technique for maintaining a substantially constant level of an amplified output signal even though the input signal level is varying. In broadcast communication receivers, automatic gain control (AGC) is employed to provide a consistent audio output level despite fluctuations in signal strength of a tuned-in broadcast signal or fluctuations occurring while retuning to other broadcast signals.
Communication receivers typically include many stages of amplification. In superheterodyne receivers, amplification is required for various frequency ranges including an RF amplification stage and an IF amplification stage. The RF amplifier stage typically has a large bandwidth of amplified frequencies while the IF amplifier has a relatively narrow bandwidth.
An AGC circuit controls the gain of the RF amplifier stage in inverse proportion to a measured signal level downstream in the receiver. The measured signal level can correspond to a wideband signal level resulting from the RF amplifier stage, from a narrowband signal level resulting in the IF amplifier stage or the detector, or a combination of both. Use of only the narrowband signal level can result in saturation of the mixer following the RF amplifier stage where strong RF signals are present at frequencies other than the frequency of the desired received signal. Use of only the wideband signal level during those same conditions results in desensitizing the receiver to the desired signal which becomes greatly attenuated.
Systems known as keyed AGC systems control the RF amplifier gain according to both the wideband and narrowband signal levels. The RF gain is controlled in accordance with the wideband signal level unless the narrowband signal level becomes too small (i.e., the receiver is becoming desensitized). In that case, the RF gain is increased (i.e., the AGC signal is reduced) to restore the narrowband signal to a usable level.
Keyed AGC systems have the disadvantage that intermodulation distortion is not removed to the same extent that it is in AGC systems employing only wideband control.